This is perhaps the most stable numeral in Proto-Sino-Tibetan, with virtually no daughter language failing to show some reflex of this root. This stability can be attributed to various factors: the stability of the consonants "s" and "m", and the saliency of the numeral THREE itself. The velar prefix g/k- is the only prefix reconstructable for this root (another manifestation of well-preservedness); this is of course disregarding other prefixes in modern languages which resulted from an innovative prefix run in all of the lower numerals, for example Jingpho (m- in 3-5).

Some languages also show /a/ vocalism (e.g. Chinese), which some dismiss (somewhat unconvincingly) as secondary development. It is very likely that ablaut of *u ~ *a existed in the proto-language, i.e. *g-sum ⪤ *g-sam, analogous to *b-suŋ ~ b-saŋ (“fragrance”) (Matisoff, 1997).